I just discovered that letter-spacing doesn't always work in webkit (and perhaps other browsers?) when embedding fonts. I've tried it on two different fonts and received different results.
"League Gothic Regular" will work
http://www.theleagueofmoveabletype.com/fonts/7-league-gothic
"Quicksand Light" will not
http://www.fontsquirrel.com/fonts/Quicksand
What's the skinny? Is there a way to force the hand of non-compliant fonts?
Quick note: Firefox does seem to apply to both fonts.